According to Piaget, two major principles guide intellectual growth and biological development: adaptation and organization. For individuals to survive in an environment, they must adapt to physical and mental stimuli. Assimilation and accommodation are both part of the adaptation process. Piaget believed that human beings possess mental structures that assimilate external events, and convert them to fit their mental structures. Moreover, mental structures accommodate themselves to new, unusual, and constantly changing aspects of the external environment. Piaget’s second principle, organization, refers to the nature of these adaptive mental structures. He suggests that the mind is organized in complex and integrated ways. The simplest level is the schema, a mental representation of some physical or mental action that can be performed on an object, event, or phenomenon.